176 CHEMICAL ENGINEERING stopping the compressor or wasting the gas into the atmosphere. Its steadiness of blast makes it also very valuable for oil burning and for general forge work. For blast furnaces, where the resistance to the flow of the air is likely to vary from time to time, while a uniform supply of oxygen or air regardless of such variation in resistance is important, the centrifugal compressor may be supplied with a constant-volume governor. This governor, actuated by the variation of air velocity in the compressor inlet, causes the speed of the driver to vary in accordance with the needs of the furnace, and thus maintains a constant volume (referred to atmospheric conditions) of air against widely varying pressures with fairly constant efficiency. The centrifugal compressor also finds wide application as an exhauster in ash conveying, sawdust conveying, and in the general pneumatic conveying of coal, cement, rice, starch, etc. For intermittent work, such as pneumatic cash and mail conveying, its prompt response to overloads allows the use of a comparatively small driver. In coke-oven-gas manufacture the centrifugal compressor maintains constant suction on the gas main, and then compresses the gas so that it will flow through the condensers, purifiers, and into the gas holders. It is frequently used as a booster to a high-pressure reciprocating compressor, the two compressors forming together a very compact and efficient set. By compressing the air, say, to 30 Ib. per square inch gage in the centrifugal compressor, the volume of the air to be handled by the reciprocating compressor is only about one-third of what it would otherwise be, making it possible to employ a much smaller unit. While a centrifugal compressor will maintain a fairly constant pressure over a wide range of quantities, there is for every speed a certain range of quantity at which the discharge vanes cease to co-operate, causing a sudden drop in pressure. This "break-down" region can be pushed back toward lighter loads, and the drop in pressure made less abrupt by making the discharge vanes very few and their inlet angle small. Also, when working on that part of the pressure curve where the pressure increases with the quantity or remains constant, there are usually pressure surges or pulsations which, while slight in themselves, may be greatly intensified by a sort of resonance effect if the volume of the inlet and of the discharge piping happens to have a certain critical value. A slight throttling of the inlet will always stop these pulsations by making the pressure curve slightly drooping. FIG. 34. THEORY Notation.—Referring to Fig. 34 let De(Da) = impeller inlet (exit) diameter, feet. ue(ua) — impeller inlet (exit) peripheral velocity, feet per second. we(wa) — absolute inlet (exit) velocity of gas, feet per second. ve(va) — relative inlet (exit) velocity of gas, feet per second. be(ba) = impeller inlet (exit) angle, degrees. de = angle between we and uc, degrees. da = angle between wa and ua, degrees, or inlet angle of discharge vanes, if any. Pi = initial pressure of the gas, including the velocity energy (if any), pounds per square inch. T! = temperature of gas corresponding to pi, degrees Fahrenheit absolute. di = density of gas corresponding to pi, pounds per cubic foot.